1. Field of the Invention
The present invention generally relates to sensing electrodes for titrations and, more particularly, to sensing electrodes for detecting the end points of potentiometric titrations of surfactants in solution.
2. State of the Art
In numerous instances, it is desirable to detect the concentration of surfactants in aqueous solutions. For example, to maintain product quality during production of commercial detergents and to assess product stability over time, it is necessary to test for the concentration of surfactants in solutions. Similar quantitation tests are required for the raw materials from which commercial detergents are compounded. Also during the formulation of new surfactant compounds and the development of new applications for surfactants, it is normally required to analytically test surfactant concentrations.
A surfactant can be broadly defined as an organic compound that encompasses two dissimilar structural groups, such as a water-soluble group and a water-insoluble group. Included within this definition are soaps and hydrotropes. The principal uses of surfactant compounds are in household detergent products. The critical characteristic of a surfactant is its surface activity including wetting and micellar solubilization. Surface activity of a surfactant molecule is determined by the composition, solubility properties, location and relative sizes of the dissimilar structural groups that make up the molecule. Various pairs of names have been used to designate the dissimilar groups: hydrophobic-hydrophilic, lipophobic-lipophilic, and oleophobic-oleophilic. Also, the terms polar and non-polar are often used to designate molecular surfactant groups that are soluble or insoluble, respectively, in water. The molecular weight of surfactants of commercial interest ranges from the low hundreds of grams per mole to a high of many thousands of grams per mole in the case of some hydrophilic polymers.
The hydrophilic or "solubilizing" groups in surfactants can be classified into four categories: anionic, cationic, amphoteric and nonionic. In anionic surfactants, the hydrophilic groups are negatively charged in aqueous solutions or dispersions. Anionic solubilizing groups include, for example, carboxylates, sulfonates, sulfates, and phosphates.
In cationic surfactants, hydrophilic groups bear a positive charge in aqueous solutions. Cationics are normally solubilized by amino groups, or quaternary nitrogens. To increase water solubility of cationic surfactants, additional amino groups can be introduced or an amino group can be quaternized with a low molecular weight alkyl group.
Amphoteric surfactants are those containing both an acidic and basic moiety in their structure. These may be any of the anionic or cationic groups, and a single amphoteric molecule may contain several ionic functions. Oxygens may also be present, as in nonionics, to strengthen the hydrophilic tendency of amphoteric molecules.
A nonionic surfactant, as the name implies, has essentially no charge when dissolved or dispersed in an aqueous medium. The hydrophilic tendency in nonionic molecules is due primarily to oxygen present in the form of alkoxy groups which interact with water molecules.
Chemical analysis of surfactants in aqueous solutions is of commercial importance and, specifically, it is often of commercial interest to determine the concentration, usually expressed in terms of moles per liter, of surfactant molecules, containing ten or more carbon atoms. The standard assay of anionic and cationic-active detergents in aqueous solutions is the two-phase Epton titration. This manual procedure is described by V. Reid, et al. in "Determination of Anionic-Active Detergents by Two-Phase Titration", Commission International d' Analyses, Milan, 1966. To determine stoichiometric endpoints in such titrations, three types of colorimetric indicators are commonly used: bromocresol green, methylene blue, and dimidium bromide-disulphine blue (DMB-DSB). Titrations with the mixed indicator DMB-DSB are described in "A Systematic Scheme of Semi-micro Qualitative Analysis for Anionic Surface-Active Agents," Holness, et al., Analyst, 1957, Vol. 82 pp 166-176. A typical stock solution of DMB-DSB is 2.63.times.10.sup.-3 molar in DMB and 4.42.times.10.sup.-4 molar in DSB and can be commercially obtained from British Drug House, Ltd., in Poole, England.
In a standard two-phase Epton titration, titrant is added to a surfactant solution until a color change in the indicator identifies the stoichiometric endpoint. It may be noted that DMB-DSB, changes color in response to either cationic or anionic surfactants. An advantage of two-phase titrations is that samples of both raw material surfactants and commercial detergent formulations can be titrated without substantial interference from other components in the samples; specifically, surfactant molecules containing ten or more carbon atoms can be titrated by two-phase titrations without substantially titrating lower molecular-weight substances or inorganic ions that are in the same sample solution. A drawback of two-phase titrations is that the time required for a typical analysis is about thirty minutes per sample. Also, because the non-aqueous phase in standard two-phase titrations is chloroform, analysts are exposed to a potential hepatotoxin when performing the assays. Further, the results of two-phase titrations with colorimetric indicators are subjective because color changes must be judged visually.
In the case of assays of anionic and cationic surfactant molecules containing more than ten carbon atoms, standard two-phase titrations are based upon the ion pairing of the surfactant molecule with a titrant of the opposite charge. For example, as measured quantities of cationic titrant are added to an aqueous surfactant solution of known volume, the cationic titrant displaces the cationic dye indicator causing a color change at the end point of the titration.
In the case of nonionic surfactants, common two-phase Epton titrations are not possible. An alternative titration technique requires the reagent 1,2-dichloroethane, which is a health hazard. As another alternative, quantitative analyses of nonionic surfactants are performed on high pressure liquid chromotography (HPLC) instruments, which also require relatively extensive sample preparation and hazardous solvents.
In view of the shortcomings of two-phase titrations of surfactants, workers in the art have attempted to devise potentiometric methods to perform the assays. In general, potentiometry requires assembling an electrochemical cell that has an indicator electrode sensitive to one of the ions to be studied and a reference electrode of fixed potential. The basic reference electrode is the standard hydrogen electrode; however, calibrated secondary reference electrodes, such as silver-silver chloride or mercury-calomel electrodes, are used in practice. In a potentiometric cell of typical design, the indicator electrode is surrounded by a solution of unknown ion concentration to which the electrode is sensitive, and the reference electrode is surrounded by a reference solution of known concentration. The electromotive force (e.m.f.) between the indicator electrode and the reference electrode of a potentiometric cell is related to ion activity which, in turn, is related to the concentration of ions in solution. More particularly, the e.m.f. across an ideal potentiometric cell (i.e., one that obeys the Nernst equation) is proportional to the logarithm of the activity of the ions in the solution to which the indicating electrode is sensitive.
In potentiometric cells for detecting concentrations of surfactant molecules, it is known to use ion-selective membrane electrodes that control ion migration between the reference solution and the test solution to be assayed. The membranes may be formed, for example, from an ion-selective glass material, an ion-selective polymeric material, or an ion-selective water-immiscible liquid. A liquid membrane electrode, for example, is described by Goina, et al. in "Potentiometric Titration of Sodium Cetyl Sulfate (NaCS) Using Liquid-Membrane Selective Ion Electrodes," Sosit la redactie, 16 April 1982. According to this reference, a suitable liquid membrane contains the solvent o-dichlorobenzene, an anionic surfactant, and one of the following dyes: crystal violet, malachite green, and methyl violet. Ion-selective polymeric membranes are also described in U.S. Pat. Nos. 3,562,129; 3,691,047; and 3,753,887. Such membranes are generally composed of a polymeric matrix in which is dispersed or dissolved a suitable cation-exchange material for a membrane that is to be sensitive to cations, or an anion exchange material for a membrane that is to be sensitive to anions.
In the case of potentiometric titrations, a titrant is added to a sample solution in an electrochemical cell to combine with ions in the solution. When the titrant is properly chosen, changes in the conductivity of the cell will be relatively abrupt at the end point of the titration. By electrometrically monitoring the e.m.f. of the cell during the titration, an inflection point on the titration curve can be identified which generally indicates the end point. In direct potentiometry, the e.m.f. between the indicator and reference electrodes is measured during titration by a suitable electrometric device.
With potentiometric titrations, automation becomes possible and analysis time can be substantially reduced. Also, potentiometric titrations minimize subjectivity in measurement because identification of colored endpoints is not necessary. However, a shortcoming of conventional potentiometric titrations is that indicating electrodes are normally responsive only to closely related homologues of the ion-exchange material bound in the membrane of the electrode. Furthermore, conventional indicating electrodes are responsive to only cationic or anionic surfactants, not both. Thus, indicating electrodes for potentiometric titrations normally must be carefully matched with the type of surfactant to be titrated and must be replaced when different types of surfactant are titrated.
A typical potentiometric cell is generally shown in U.S. Pat. No. 4,597,848. According to that patent, the indicating electrode is of the barrel-type and comprises a hollow body having a bottom closed by an ion-exchange membrane. The ion exchange membrane is formed of a solid polymer membrane of an anion-exchanger or a cation-exchanger, or may be amphoteric. A stable electrode, such as a calomel electrode, and the indicating electrode are immersed in an aqueous sample solution whose ion concentration is to be measured. Migration of ions between the sample solution and the solution within the indicating electrode is controlled by the ion-exchange membrane. The patent states that the indicating electrode with particular ion-exchange membranes is suitable to determine the activity of chloride ions in biological solutions. Another potentiometric cell using a barrel-type indicating electrode sensitive to amines is described in "PVC-Surfactant-Selective Electrode Responsive to Primary Amines," Colloids and Surfaces, 15 (1985) 277-283.
Ion-selective polymeric membranes are also discussed in "Film Ionselective Alkylsulfate Electrodes Based On Quaternary Ammonium Salts," A. Gulevich, et al., Journal of Analytical Chemistry, Vol. XL (1985), No. 9. The article describes a membrane electrode having a polyvinyl chloride (PVC) casing with adhesive electrode-active film consisting of PVC, dibutyl phthalate and alkylsulfate salts of trinonyl octadecyl ammonia as electro-active substances. According to the article, the electrode is reversible with respect to surfactants that include alkylsulfate anions.
For potentionmetric titrations of nonionic surfactants, ion-selective polymeric membranes are described in "Barium-Polyethoxylate Complexes and Potentiometric Sensors and Their Application to the Determination of Non-ionic Surfactants," D. Jones et al, Analyst, September, 1981, v. 106, pp. 974-984, and in USSR Author's Certificate No. 1,078,325A to V. Ivanov, et al. According to the latter publication, a membrane for a barrel-type electrode is formed of a matrix containing a salt of an anion and cationactive surfactant or nonionic surfactants complex with tetraphenylborate and barium cations. To make the electrode sensitive, a mixture of nonionic and cationic surfactants was used. Preparation and application of nonionic surfactant-selective electrodes is also described in an article by Qian Xixing, et al. appearing in Fenxi Huaxue, Vol. 13, No. 5 (1985), pp. 383-385.
An electrode selective only to organic cations is described in Japanese Patent Application No. 58-201240 disclosed 25 May 1985. According to that patent, the electrode is made from hydrophobic esterified or etherified dextrins dissolved in a polymer such as PVC containing a plasticizer such as dioctyl phthalate, dioctyl adipate, and tricresyl phosphate. The patent states that such electrodes are responsive to organic cations and some quaternary ammonium salts. The disclosed membrane material is said to be useful for both barrel-type electrodes and "coated-wire" electrodes.
In a coated-wire electrode, an ion-selective polymeric membrane is formed as an outer layer, or coating, on a conductive substrate such as a metallic wire. Such electrodes are discussed in "Surfactant-Sensitive Polymeric Membrane Electrodes," S. F. Cutler, et al., Journal Electroanalytical Chemistry, 1977, pp. 145-161, which describes the testing of a coated-wire electrode consisting of a platinum wire coated only with a thin film of plasticized PVC. According to the article, when the PVC did not contain any identifiable ion-exchange compound, the potentials of the coated-wire electrode responded to increasing concentrations of either cationic or anionic surfactants, but were irreproducible. The article reported that reproducible potentials were not obtained by incorporating an ion-exchange complex into the polymeric coating but were achieved when a dissolved plasticizing complex of a cationic and an anionic surfactant was incorporated into a PVC or polyvinyl bromide (PVB) matrix membrane. Specifically, the article reports that coated-wire electrodes providing reproducible potential measurements were made with PVC and PVB membranes plasticized with 40-60% tricresyl phosphate (TCP) or a pentaerythritol ester. It was further reported, however, that such electrodes became unstable or insensitive with prolonged usage (e.g., when immersed in concentrated surfactant solutions for periods exceeding six hours). The loss of stability was ascribed to solubilization of the ion-exchange complex. According to the article, solubilization was prevented by modifying the electrodes to have fixed charges chemically bound to the polymers and, specifically, by modifying the plasticized polymer membranes to contain cetyl trimethylammonium dodecyl sulphate. Such electrodes, referred to as chemically-modified polymer membrane electrodes, were said to react to either anionic or cationic surfactants according to the particular modifications made to the polymer.
Subsequent to the Cutler, et al. article, supra, it was reported in "Determination of Anionic-active Matter in Detergents by Potentiometric Titration," G. C. Dilley, Analyst, July, 1980, Vol. 105, pp. 713-719, that coated-wire electrodes as described by Cutler, et al. could be used to potentiometrically titrate a number of different anionic surfactants and that the electrodes provided potentiometric curves for which the titration endpoints could be reliably found. As indicated by its title, the article discussed efforts to adopt potentiometric titrations to detergents. Specifically, the article describes use of an ion-selective membrane in conjunction with a barrel-type electrode to potentiometrically indicate the endpoints of titrations of anionic-active matter in simple detergent solutions. For such usage, a suitable membrane was reported as containing only high relative mass PVC (40%) and tricresyl phosphate (60%) conditioned with a solution of sodium lauryl sulphate (SLS).
Coated-wire electrodes are also reported in Czechoslovakian patent No. 225,222 published December, 1984. According to an example provided in the patent, an ion-selective membrane was coated onto a conductive wire by repeatedly dipping the wire into a solution of PVC (85 mg), di-n-octyl phthalate (0.2 ml) and Reinecke acridine orange (0.5 mg) in the solvent tetrahydrofuran (THF). After each dipping, the solvent was evaporated until the conductive wire was covered with a membrane of predetermined thickness.
General operation of ion-selective coated-wire electrodes, as well as their application to various analytical problems involving organic species that are anionic or cationic, are discussed by L. Cunningham and H. Freiser in Analytica Chimica Acta, 180 (1986) 271-279. The authors particularly describe coated-wire electrodes for sensing high-molecular weight protonated amines in the physiologic pH range. According to the article, a coated-wire electrode was made by dipping the exposed end of an insulated wire into a solution containing an amine, 5% PVC in the solvent tetrahydrofuran (THF), 0.5% in dinonylnaphthalene sulfonic acid (DNNS), a lipophilic anionic extract, and 4.5% in plasticizer, usually dioctylphthalate. The coated-wire electrodes were said to be ready for use following conditioning in a 10.sup.-4 to 10.sup.-3 M solution of the analyte.
In U.S. Pat. No. 4,399,002 to Freiser, et al., a coated-wire electrode is described that is sensitive to large organic cation species in aqueous test solutions. Specifically, the patent states that the membrane for the coated-wire electrode is composed of a polymeric matrix in which is dispersed or dissolved a cation exchange material whose counter-anion is said to be a high molecular weight alkyl or alkaryl sulfonate or sulphate of the formula R(O).sub.n SO.sub.3 wherein n is 0 or 1, R is, for example, an alkyl group having at least 13 carbon atoms. According to the patent, such a membrane will generally include cation exchange material in amounts ranging from about 1% to about 25% by weight, and a plasticizer compatible with both the polymer and the cation exchange material in amounts ranging from about 10% to about 50% by weight of the membrane.
Further according to the patent, a suitable membrane may be prepared from a homogeneous solution of the cation exchange material, the polymer, and the optional plasticizer in an organic solvent such as an alcohol, a ketone, an ester, or a cyclic ether such as THF.